As gas and oil production progress in a given field or well, the ratio of produced hydrocarbon to produce salt water, or brine, increases. Also, water injection for pressure maintenance or secondary recovery procedures can cause large volumes of brine to be produced. In the continental United States it is estimated that, on average, between 15 and 20 barrels of brine are produced for every barrel of oil, or gas equivalent. One of the major problems associated with this produced brine is scale formation.
The three most common types of scale to form in the gas and oil industry are calcite, the calcium sulfates and barite. The present invention relates to controlling the formation of calcite scale.
Calcite crystals are composed mostly of calcium carbonate (CaCO.sub.3), but often contain up to 20% of iron or magnesium carbonate. Although naturally occurring calcite, such as Iceland spar, is often essentially pure calcium carbonate, the scale formed from flowing brine generally contains several percent iron. This coprecipitated iron is often from corrosion products deeper in the well, but also can be the result of naturally occurring siderite or other materials in the production formation. Calcite scale formation is generally a consequence of the pressure drop that accompanies production. Simply put, this pressure drop removes carbon dioxide from solution and, because aqueous carbon dioxide is essentially carbonic acid, this removal of carbon dioxide increases the solution pH and causes calcite precipitation. Also, there is a secondary consequence of the pressure drop: the inherent solubility of calcite in salt water decreases as the pressure decreases. Both of these effects tend to cause calcite to precipitate during production.
Although scale can be eliminated by controlling flow rate, pressure, temperature, or by inducing precipitation or acid addition, these methods can negatively impact production and are generally less cost effective than using threshold chemical scale inhibitors. Threshold scale inhibitors are special chemicals which catalytically prevent solid particles from forming, even though the brine is sufficiently supersaturated with respect to calcite that scale formation would otherwise begin in minutes. These chemicals are referred to as "threshold" scale inhibitors because they prevent nucleation and scale formation at concentrations which are far too low to be effective by reacting with calcium ions in solution such as occurs with chelating agents. The most common methods of introducing threshold scale inhibitors to the oil or gas well are: (1) to "squeeze them into the oil or gas formation and then let them slowly release as production resumes, and (2) to continuously pump the threshold scale inhibitor into brine with a small metering pump.
Hundreds of proprietary chemicals in blends are available as threshold scale inhibitors. These proprietary chemicals are generally composed of materials which fall into one of three chemical classes: (1) low molecular weight polycarboxylates, including polyacrylates and polymaleates; (2) inorganic polyphosphates and phosphate esters; and (3) phosphonates. Each class of chemicals has properties which make certain materials desirable depending on specific conditions.
The notion of a scale inhibitor squeeze is to push or squeeze a solution containing scale inhibitor into the producing gas or oil well and fix the scale inhibitor in the producing formation. When production is resumed, the threshold scale inhibitor will be produced back in the water phase at a concentration sufficient to inhibit scale, in this case, calcium carbonate.
An ideal scale inhibitor squeeze would take no time to complete, require no equipment, have a zero chance of causing damage to the oil or gas formation, and the scale inhibitor in the first barrel of produced water produced after the squeeze would be at the optimum concentration to inhibit scale with no wasted scale inhibitor. Most importantly, the ideal scale inhibitor squeeze procedure would never have to be repeated. Short squeeze life requires the entire squeeze procedure to be repeated more often than is necessary, and over the life of a producing well, can result in tens of thousands to millions of wasted dollars. Research in recent years regarding inhibitor squeeze design has vastly improved effectiveness and lifetime.
Generally, a squeeze is performed and scale inhibitor flows back at low concentrations which increase and peak rapidly to some value and then decline within a few days to a plateau concentration which comprises the bulk of the squeeze duration. It is required that this plateau value is sufficient to inhibit scale. The more efficient the inhibitor in terms of active concentration, the more likely the plateau value will be acceptable and the longer the squeeze will last. Generally, squeeze designs consist of pumping three solutions--the preflush solution, the pill solution which contains the threshold scale inhibitor, and the overflush solution. In addition, it is usually desirable to shut-in the well for some time after the squeeze treatment to allow the threshold scale inhibitor to fix itself or equilibrate in formation.
There is little agreement in the art regarding the primary mechanism by which the threshold scale inhibitor fixes itself in the producing oil or gas well formation as a result of the squeeze procedure. The fixation mechanism of the threshold scale inhibitor determines the ultimate squeeze design. The source of disagreement in the art centers on whether the fundamental fixation mechanism of the threshold scale inhibitor is adsorption or precipitation. One school of thought considers that inhibitors are retained by adsorption onto the producing oil or gas well formation materials, thus when production is resumed after the squeeze procedure, the inhibitor desorbs into the produced water. A second school of thought considers that the threshold scale inhibitor is precipitated, probably as a complex calcium salt, in the oil or gas well during the squeeze, thus under this mechanism when production is resumed, the inhibitor is slowly dissolved into the produced water.
It is an object of this invention to provide a method for threshold scale inhibitor squeeze application in an oil or gas well that accommodates fixation of the threshold scale inhibitor in the gas well by either an adsorption mechanism or a precipitation mechanism.
This and other objects and advantages of the present invention are described, in and will be apparent from, the detailed description of the presently preferred embodiment discussed below.